Co-seismic grace gravity-based 11-layered 3-D thrust fault model for the Sumatra earthquake 2004

Abstract

Our co-seismic Gravity Recovery and Climate Experiment gravity data (level 2 ‘RL_05’ data product ‘GX-OG-_2-GSM) for the Sumatra earthquake 2004 is obtained by differencing monthly gravity field average for November 2004 from that of January 2005 and band-pass filtering (\(17{-}30^{\circ }\) and orders) in the spectral domain. Here, we propose a 11-layered three-dimensional (3-D) thrust fault gravity model based on different co-seismic rupture models in the literature. It honours co-seismic deformation of the ocean surface, ocean bottom and subsurface earth medium, different earthquake parameters and hypocentre information (\({\sim }35\) km below mean sea level). Our forward gravity response matches well with the observed gravity (RMS error of \(0.06257\,\upmu \hbox {gal}\) (6.26%)) data and our model allowed an independent computation of rupture length, instantaneous velocity, average seismic moment and momentum, which are, respectively, 1560 km, 2.9 km/s, \(4.53\times 10^{22 }\,\hbox {N}\,\hbox {m}\) and \( 9.7\times 10^{17}\,\hbox {kg}\,\hbox {m/s}\). These parameters fairly agree with those in the literature. The computed momentum indeed corresponds to an area pulse (\(9.7\times 10^{17}\,\hbox {kg}\,\hbox {m/s}\)) at ocean bottom that led to a tsunami generation. Thus, the proposed multi-layered 3-D gravity model in traditional fashion fully accounts for co-seismic gravity signal of the Sumatra earthquake 2004.